Nearly all roadways are posted with physical speed limit signs. The speed limits may be set according to a wide variety of rules and by different entities. Countries or states may set speed limits for interstates or major highways, and municipalities such as cities and towns may set speed limits for smaller roads and streets. The speed limits may be selected under rules based on the curvature or lane width of the road. The speed limits may be selected under rules based on the proximity to urban or rural areas. Other considerations such as school zones, bridges, or pedestrian crossings may impact the selection of speed limits. Because of these variable situations, it is not possible to reliably identify the speed limit of a road from the placement of the road or shape of the road on the map. Instead, posted speed limits are detected in order to identify speed limits of roads on the map. Other indicia or road postings may be similarly inconsistent.
The speed limits or other indicia may be posted using a variety of techniques. Some speed limits may be posted on a sign driven into the ground adjacent to the road. Other speed limits may be posted on overhead signs. Some speed limits may be constant values painted on the signs. Other speed limits may be variable and displayed electronically. The locations and types of speed limit signs are not consistent among groups of roads or even along the same road. Challenges remain in detecting speed limit values or other sign indicia reliably.